The invention relates to a process for passing carbon dioxide through a cased, cemented and perforated well. Carbon dioxide in the presence of moisture leads to carbonic acid attack of oil well cement. This attack, if allowed to proceed unchecked eventually weakens the cement to an unacceptable condition. The major constituent of set cement, hydrated calcium silicate breaks down by the action of CO.sub.2 as follows: EQU 3Ca0.2SiO.sub.2.3H.sub.2 O+3CO.sub.2 .fwdarw.3CaCO.sub.3 +2SiO.sub.2 +3H.sub.2 O.
The reaction products calcium carbonate and silica possess less binding power than hydrated calcium silicate. Therefore, a considerable deterioration in cement strength takes place by CO.sub.2 attack.
Cement is exposed to CO.sub.2 in wells for injection of CO.sub.2 and in those used for the production of reservoir fluid which, at a certain stage of a CO.sub.2 -flooded project, contains CO.sub.2 as well. Also in source wells, used for the supply of CO.sub.2, there is exposed cement.
The only place of exposure of cement to CO.sub.2 in injection, production and source wells is at the perforations, where the perforation tunnels traverse the cement sheath between casing and borehole. To maintain the integrity of injection, production and source wells, the exposed cement should be shielded from CO.sub.2 attack. This can be achieved by providing the exposed cement with an impermeable layer of inert material which protects the cement. This protective layer applied on the cylindrical walls of the perforation tunnels, should be strong enough to resist high shear rates normally occurring in well perforations, but yet thin enough to prevent injectivity or productivity impairment. It has now been found that epoxy resin is very suitable for this purpose. The invention therefore relates to a process for passing carbon dioxide through a case, cemented and perforated well, characterized in that the carbon dioxide is passed through perforation tunnels in the cement which have been internally coated by epoxy resin.